1. Field of the Invention
The present invention relates generally to a surface acoustic wave device. More specifically, the invention relates to a resonator type surface acoustic wave device utilizing a longitudinal mode and operative in a frequency range of a VHF band and a UHF band.
2. Description of the Related Art
In a resonator type surface acoustic wave device, fundamental mode of a mode where energy is distributed perpendicularly to propagating direction of a surface acoustic wave (hereinafter referred to as transverse mode), is a primary propagation mode. In certain case, a transverse secondary mode becomes spurious.
For restricting the transverse secondary mode to be spurious, there is a technology to narrow an aperture with W0 of an interdigital electrodes 91 and 92 and grating reflectors 4 and 5 for cutting off the transverse higher mode spurious, as shown in FIG. 8. It should be noted that the reference numeral 10 denotes a piezoelectric substrate.
Another technology is to provide weighting of COS.sup.-1 function for electrode length of the interdigital electrode 81, as shown in FIG. 9.
A further technology has been disclosed in Japanese Unexamined Patent Publication (Kokai) No. Showa 62-219709, in which is proposed a method to set a ratio W/W0 of a overlap width W versus the aperture width W0 of the interdigital electrode in a range of 0.65 to 0.75.
In the technologies simply minimizing the aperture width W0 as shown in FIG. 8 and providing COS.sup.-1 function weighting as shown in FIG. 9, a configuration of the interdigital electrode is restricted to make it difficult to obtain desired characteristics of the surface acoustic wave device and thus to restrict design thereof.
Even when the technology disclosed in Japanese Unexamined Patent Publication No. Showa 62-219709 is employed, little restriction effect for the transverse secondary spurious mode can be obtained in the surface acoustic wave device on a piezoelectric substrate having small electromechanical coupling factor, such as a quartz substrate or so forth.
The reason is as follows. In the substrate having small electromechanical coupling coefficient, a difference between a velocity of the surface acoustic wave propagating in the interdigital electrode and a velocity of the surface acoustic wave propagating a portion other than inside of the interdigital electrode becomes small to weaken cooping of transverse mode energy within the interdigital electrode.
At this time, distribution of the transverse mode spreads transversely perpendicular to the propagating direction of the surface acoustic wave. Then, simultaneously with shifting of a peak position of distribution of the transverse secondary mode (see 22 of FIG. 2) out of the interdigital electrode, the aperture width for canceling transverse secondary mode excitation becomes large.